The invention is directed, in general, to video signal processing and, more specifically, to a system and method for color-compensating a video signal.
This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is, or what is not, prior art.
Image or video display systems have been, and continue to be, important devices for presenting visual information. A video display system may take various forms, including a cathode ray tube or a flat-panel display such as a liquid crystal display (LCD) or a plasma display panel (PDP). A video display system may also take the form of a front or rear video projector, which employs a white light source or one or more lasers or light-emitting diode (LEDs) as colored light sources and may include a spatial light modulator (SLM), such as an LCD or a digital mirror device (DMD), to modulate light emanating therefrom. A video signal bearing an ordered sequence of frames is provided to a video display system to cause it to produce a still or moving image. The video signals may be analog or digital and may be encoded according to any one of a variety of standards.
Color video encoding standards define a reference white to exist at a certain temperature and primary colors (usually three) to exist at certain CIE (Commission Internationale d'Eclairage) color coordinates. Standards often define three primary colors that appeal to human eyes: red, green and blue (RGB). Some standards use more than three primary colors. Irrespective of their number or their color coordinates, the primary colors inherently define a “colorspace” within which all colors in all images encoded according to the standard must lie.
As those skilled in the pertinent art understand, video display systems are physical devices and therefore act in accordance with the properties of the materials they use and the physical principles that underlie their operation. These properties and principles skew to some extent the color coordinates of the primary colors they produce. Consequently, the image that a given video display system produces varies in color, or chrominance, from the image the video signal directs. To complicate matters, video display systems may respond nonlinearly to variations in the driving force (e.g., voltage) directly derived from the video signal, causing variations in luminance from what the video signal directs. Further, different types of video display systems use different materials and employ different physical principles and therefore reproduce different images from the same video signal.
A video signal should be precompensated to counteract video display system response. One type of precompensation is directed to counteracting variations in luminance and is called gamma-encoding (also called gamma compensation or gamma compression).
Another type of precompensation counteracts variation in chrominance. The International Electrotechnical Commission (IEC) has issued a standard, 61966-2-4:2006(E) (incorporated herein by reference in its entirety), that sets forth a chrominance precompensation procedure in which a gamma-encoded video signal containing luminance and chrominance components (YCrCb) based on standard primary colors is: (1) transformed into a gamma-encoded signal in a standard RGB colorspace (the signal being referred to as an R′G′B′ signal, the primes denoting that the signal is gamma encoded), (2) then gamma-decoded in the RGB colorspace (the signal then being referred to as an RGB signal), (3) then transformed into a chrominance-compensated colorspace, called rgb, defined by the color coordinates of the video display system's primary colors (the signal then being referred to as an rgb signal), (4) then gamma-reencoded in the rgb colorspace (the signal then being referred to as an r′g′b′ signal) and (5) then finally transformed into a format suitable for the video display system hereinafter called a “system-native format”. Transformations (1) and (5) are usually linear. Transformations (2) and (4) are nonlinear. Transformation (3) is linear.
To address the above-discussed deficiencies of the prior art, one aspect of the invention provides a system for color-compensating a video signal. In one embodiment, the system includes: (1) a first transformation circuit configured to receive and transform a gamma-encoded input video signal into a gamma-encoded RGB video signal R′G′B′ and (2) a second transformation circuit coupled to the first transformation circuit and configured to receive and linearly transform the gamma-encoded RGB video signal R′G′B′ into a chrominance-compensated, gamma-encoded rgb video signal r′g′b′. The first and second transformation circuits may be combined into a single transformation circuit that performs a single linear transformation.
Another aspect of the invention provides a method of color-compensating a video signal. In one embodiment, the method includes: (1) transforming a gamma-encoded input video signal into a gamma-encoded RGB video signal R′G′B′ and (2) linearly transforming the gamma-encoded RGB video signal R′G′B′ into a chrominance-compensated, gamma-encoded rgb video signal r′g′b′. The transforming and the linearly transforming may be carried out in with a single transform.
Yet another aspect of the invention provides a video display system. In one aspect, the system includes: (1) an input configured to receive a gamma-encoded input video signal, (2) a first transformation circuit configured to receive and transform the gamma-encoded input video signal into a gamma-encoded RGB video signal R′G′B′, (3) a second transformation circuit coupled to the first transformation circuit and configured to receive and linearly transform the gamma-encoded RGB video signal R′G′B′ into a chrominance-compensated, gamma-encoded rgb video signal r′g′b′ and (4) a third transformation circuit coupled to the second transformation circuit and configured to receive and transform the chrominance-compensated, gamma-encoded rgb video signal r′g′b′ into a system-native luma-chroma-chroma format. (Luma is used herein to designate gamma-compensated luminance, and chroma is used herein as a synonym of chrominance.)
For a more complete understanding of the invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The chrominance precompensation procedure of IEC 61966-2-4:2006(E) counteracts system chrominance variation and is therefore regarded as a substantially exact solution. However, the many and varied transformations it requires are computationally intensive, requiring a large hard-wired logic circuit, a powerful processor or both. Such circuits or processors are relatively large and power consumptive.
What is needed in the art is a less computationally intensive way to color-compensate a video signal. What is also needed in the art is a way to color-compensate a video signal that perhaps allows logic circuit or processor size to be reduced and perhaps reduces the amount of power consumed performing such compensation.
In a yet more specific embodiment, the LCD video projector employs several lasers or LEDs as colored light sources and one or more associated drivers that provide power or control to the light sources. An example of a driver suitable for the yet more specific embodiment can be found in U.S. patent application Ser. No. [Attorney Docket No. G. Chen 14-1-24], filed by Gang Chen, David A. Duque, and Roland Ryf on even date herewith, entitled “Time Division Multiplexing a DC-to-DC Voltage Converter” and incorporated herein by reference in its entirety. Since lasers or LEDs produce coherent light, speckle, which degrades projector performance, may result. Accordingly, in another, more specific embodiment, the LCD video projector employs a diffuser or one or more other optical components to reduce speckle. An example of such a diffuser can be found in U.S. patent application Ser. No. [Attorney Docket No. G. Chen 12-22], filed by Gang Chen and Roland Ryf on even date herewith, entitled “Diffuser Configuration for an Image Projector” and incorporated herein by reference in its entirety.
In a still more specific embodiment, the LCD video projector is battery- or wall-plug powered and configured to produce enhanced brightness with the color compensating system 130 embodied as an IC in the integrated driving/control circuit of the LCD projector. An example of an LCD with enhanced brightness can be found in U.S. patent application Ser. No. [Attorney Docket No. G. Chen 13-23], filed by Gang Chen and Roland Ryf on even date herewith, entitled “Multi-Color Light Source” and incorporated herein by reference in its entirety. In yet another specific embodiment, the color compensation system 130 is embodied in an integrated, SLM-based video projector. Integrated, SLM-based video projectors are described in general in U.S. patent application Ser. No. 11/713,207, filed by R. Giles, et al., on Mar. 2, 2007, entitled “Direct Optical Image Projectors” and incorporated herein by reference in its entirety.
Although not necessary, the battery-powered, IC-embodied LCD video projector may be part of a larger, battery-powered device, such as a personal digital assistant (PDA), an audio (e.g., MP3) player, a digital camera or a cell phone.
As described above, standards-based chrominance precompensation procedures, while substantially exact, are often computationally complex. The disclosure is directed in general to reducing computational requirements. As a result, the size of the hard-wired digital logic or processor required to perform color compensation may be reduced. Further, the power required to perform such color compensation may be reduced. These are potentially advantageous in the context of the battery-powered devices listed above and other such devices. In general, reduced logic circuit or processor size yield lower manufacturing cost, and reduced power requirements yield lower heat dissipation, so such color compensation may find significant advantage in a wide variety of larger, non-battery-powered conventional or later developed video display systems.
In general, the color compensation system 130 transforms the video signal emanating from the video signal source 110 into a system-native format that is both gamma-encoded and at least approximately precompensated for any variations in chrominance that the remainder of the video display system 140 may contain.
As described above, IEC 61966-2-4:2006(E) sets forth a chrominance precompensation procedure in which a gamma-encoded video signal is transformed into an R′G′B′ colorspace, is then gamma-decoded into a RGB colorspace, is then transformed into a chrominance-compensated rgb colorspace, is then gamma-reencoded into a chrominance-compensated r′g′b′ colorspace and is finally transformed into the system-native format. This procedure yields a substantially exact solution, which those skilled in the art prefer for accuracy of color rendition.
However, it has been found that the computational requirements of the substantially exact, standards-based procedure may be significantly reduced without a concomitant significant loss in accuracy of color rendition by eliminating the gamma decoding and subsequent encoding steps. Those skilled in the pertinent art know that, while gamma may vary from one type of video display system to another or one particular video display system to another, it is always a nonlinear function. Thus, gamma encoding and decoding require nonlinear transforms (sometimes carried out by means of lookup tables) and are therefore responsible for a significant portion of the overall computational requirements of the standards-based chrominance precompensation procedure. It has therefore been found that color compensation can be carried out with approximate, but typically highly acceptable color rendition accuracy, in the rgb colorspace. A single linear transform can achieve such an approximate color compensation.
and ITU-R BT.709-5 relates R′G′B′ to YCC709 as follows:
For input video signal that does not follow the above mentioned standard, the video signal can be transformed into R′G′B′ using other specific transformations. The input video signal may already be a gamma-encoded signal R′G′B′ in RGB space, in which case the above transformation would not be carried out. The input video signal may not be digital, but rather a composite analog video signal, for example. In such case, the analog video signal would be digitized before being transformed into R′G′B′.
A second transformation circuit 134 is coupled to the first transformation circuit 132 and is configured to receive and linearly transform the gamma-encoded RGB video signal R′G′B′ into a chrominance-compensated, gamma-encoded rgb video signal r′g′b′. The general form for this second linear transformation is:
The values of ai,j i=1,2,3,j=1,2,3 depend upon specific system characteristics, namely the distances separating the system's primary colors from those of the standard. For example, if the system's primary colors are the same as those of the standard (the distances separating them are 0), all ai,j equal 1. Those skilled in the pertinent art are able to determine ai,j given R′, G′, B′, r′, g′and b′.
It is important to note that no material transformation circuits or transformation processes exist or are undertaken between the first and second transformation circuits 132, 134. It is also important to note that, if the transformation performed by the first transformation circuit 132 is a linear transformation, the first and second transformation circuits 132, 134 may be combined into a single transformation circuit that performs a single linear transformation that directly transforms the gamma-encoded input video signal R′G′B′ into color-compensated r′g′b′. A broken-line box (unreferenced) surrounding the first and second transformation circuits 132, 134 represents this possibility.
A third transformation circuit 136 is coupled to the second transformation circuit 134 and is configured to receive and transform the chrominance-compensated, gamma-encoded rgb video signal r′g′b′ into a system-native format for the benefit of the remainder of the video display system, as shown. In the illustrated embodiment, the system-native format is a luma-chroma-chroma format (hereinafter called ycc), meaning that the transformation performed by the third transformation circuit 136 is a linear transformation. For example, ITU-R BT.601-6 relates ycc601 to r′g′b′ as follows:
and ITU-R BT.709-5 relates ycc709 to r′g′b′ as follows:
Those skilled in the art understand that, if the system-native format is other than ycc, other specific transformations exist to transform from r′g′b′ into the system-native format.
If the transformation performed by the third transformation circuit 136 is a linear transformation, the first, second and third transformation circuits 132, 134, 136 may be combined into a single transformation circuit that performs a single linear transform that directly transforms the gamma-encoded input video signal into the system-native format. The broken-line box (unreferenced) surrounding the first and second transformation circuits 132, 134 may extend to encompass the third transformation circuit 136, as shown, and represents this further possibility. The third transformation circuit 136 is unnecessary if the system-native format is r′g′b′.
In a step 450, the chrominance-compensated, gamma-encoded rgb video signal r′g′b′ is transformed into a system-native format. In one embodiment, the system-native format is a digital, luma-chroma-chroma format (ycc). In a step 460, the system-native format is employed in a video display system to form an image. In one embodiment, the video display system is a battery-powered LCD projector. The method ends in an end step 470.
The above-described methods may be performed by various conventional digital data processors or computers, wherein the computers are programmed or store executable programs of sequences of software instructions to perform one or more of the steps of the methods, e.g., steps of the method of
Those skilled in the art to which the invention relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of the invention.